This paper investigates the effects of cetane number (CN) on energy balance between a light duty (1.9L) and medium duty (4.5L) compression ignition engine at 1500 rev/min (RPM) and nominally 1.88 bar brake mean effective pressure (BMEP) (low load) and 5.65 BMEP (medium load) conditions. The preliminary results show that CN significantly affects the distribution of supplied fuel energy in the engine systems. Increasing CN decreases heat loss to cylinder wall on both engines and load conditions. The sensitivity of increasing CN on heat loss to cylinder wall changes by various loads: high CN has stronger impacts on medium duty engine at low load condition, but results larger difference on light duty at medium load condition, which are balance results between temperature difference and combustion duration. Moreover, as CN increases, energy loss to exhaust increases on both engines at low load condition. Increasing CN increases on medium duty engine, but decreases on light duty engine at medium load condition, which are mainly impacted by changing exhaust temperature and various exhaust species. Finally, high CN consistently increases brake fuel efficiency at the low load condition for both engines. Increased CN increases brake fuel efficiency for the light duty engine at medium load, but causes a slight decrease in brake fuel efficiency for the medium duty engine at the same CA50 condition. Key words: cetane number, diesel engine, energy balance, combustion, efficiency.